1. Field of the Invention
The present invention relates to a wrist structure of an industrial robot which is provided with wrist elements which have three degrees of freedom of rotating motion.
2. Description of the Related Art
In the past, a wrist structure of an industrial robot has been known, which includes a first wrist element, a second wrist element which is supported at a front end part of the first wrist element rotatably, and a third wrist element which is supported at a front end part of the second wrist element rotatably, wherein the powers of the second wrist motor and the third wrist motor which are provided at the first wrist element are transmitted through hypoid gear sets which respectively have pinion gears and ring gears to the second wrist element and the third wrist element. For example, in the wrist structure which is described in Japanese Examined Patent Publication No. 4233578 (JP4233578B), the second wrist motor is arranged at the front end side of the first wrist element than the third wrist motor, and the power of the second wrist motor is input through a set of spur gears to a second wrist pinion gear.
In this regard, in order to drive the second wrist element, a drive torque which is larger than that for driving the third wrist element is required. For this reason, in general, the reduction ratio of the power transmission path from the second wrist motor to the second wrist element is set large. However, in the wrist structure which is described in JP4233578B, the power of the second wrist motor is input through only one set of spur gears to the second wrist pinion gear, so to obtain a sufficient reduction ratio, the hypoid gear set has to be set large in reduction ratio. If making the hypoid gear set large in reduction ratio, there is the problem that the transmission efficiency of the power falls.